1. Field of the Invention
The present invention relates generally to a transmissive-type display device (hereinafter called "transmissive display device"), and more particularly to a dot-matrix type display device having a display panel such as a liquid crystal panel and multiple picture elements arranged in a matrix, wherein the display panel is provided with an array of microlenses. This type of display device is particularly applicable to a large-screen projection TV, information display systems and the like.
2. Description of the Prior Art
In the transmissive display panels, non-linear devices such as varisters or MIM (metal insulator metal) are provided in correspondence to the picture elements so as to prevent possible cross talk between the adjacent picture elements. Switching elements such as TFTs (Thin Film Transistor) disposed for the respective picture elements drive picture element their electrodes. These elements and wiring thereto occupy a space in the display device, thereby reducing the effective area for forming picture elements. This reduces the numerical aperture of the display device. The numerical aperture ratio is expressed by (the effective area of all the picture elements) divided by (total display area).
As is evident from the formula, as the areas which do not contribute to displaying increases, the numerical aperture ratio decreases. The reduced numerical aperture ratio leads to the reproduction of dark picture, and the poor image quality.
In order to refine pictures on display, another requirement is to minimize the picture elements on the display panel. If the sizes of the components of all the picture elements are proportionally reduced, the numerical aperture ratio will not change. However, photolithograph and etching has a limit to the minimizing of the non-linear elements and switching elements, and the wiring thereto cannot be made narrow in its width below an allowable extent. As the spaces are occupied by these elements, the numerical aperture ratio of the display device is reduced.
The numerical aperture ratio is the ratio of incident rays to the display panel to a recoverable light. Unrecoverable light is shut out by an untransmissive portion of the display panel, and does not contribute to displaying. Consequently, the quality of pictures on display depends on the numerical aperture ratio of the display devices when they employ the same source of light; that is, the reduced numerical aperture ratio leads to the reproduction of a dark picture.
To solve the problems pointed out above, one proposal is for providing the display panel with an array of microlenses on one side or on both sides thereof, which are disclosed in Japanese Laid-Open Patent Publications No. 60-262131 and No. 61-11788. An advantage of these known display devices is that the incident rays to the portions of display panel that do not contribute to displaying are focused on the picture elements in the display panel, thereby increasing the numerical aperture substantially.
However, these known display devices have the following disadvantages:
In a display device whose display panel has an array of microlenses only on the light-incident side, that is, the side where light enters the display panel, after the rays are focused on the picture elements, they are diverged. To use this display panel in a projection type display device, it is required that the numerical aperture (NA) of the projection lens on the light-outlet side is deliberately enlarged.
In a display device whose display panel has arrays of microlenses on both sides, the following problems arise:
Referring to FIG. 3, a typical conventional display panel will be described in detail:
The display panel 3 has microlenses 1 and 2 on both sides. The microlenses 1 and 2 are arranged such that their focal lengths are the same. In addition, the focal points of the microlenses 1 on the light-incident side of the display panel 3 and the microlenses 2 on the light-outlet side thereof correspond to each other. As a result, the focal points of these microlenses 1 and 2 are disposed in the cross-sectional center of the display panel 3. The incident parallel rays of the microlens 1 to the light-incident side of the display panel 3 are focused on the focal points, and thereafter tend to be diverged. However, they are changed into parallel rays by passing through the microlenses 2.
The state shown in FIG. 3 is a hypothetical situation only achievable if idealistic microlenses are produced. However, the reality is that some of the incident rays do not pass through the respective microlenses 2 but pass through other microlenses due to the aberration of them. As a result, the outlet rays are diverged at angles widened by the microlenses through which they pass, as shown by the dotted lines in FIG. 3. This diversion results in the loss of light. In the case of a projection type display device, a relatively large numerical aperture will be required for the projection lens so as to prevent the light from being lost.